Known robots, such as disclosed in JP 8-47878 A and JP 2002-172572 A, transmit the power of a driving source to a robot arm by a gear type transmission mechanism, namely, a gear train. The gear type transmission mechanism of a robot of this kind includes a gear mechanism. The gear mechanism includes gears and a support member rotatably supporting the gears.
FIG. 18 is a schematic plan view of a known gear mechanism 2 having three meshed gears 3a, 3b and 3c. FIG. 19 is a sectional view taken on the line S1-S1 in FIG. 18. The gear mechanism 2 has the two stationary gears 3a and 3b, and an idle gear 3c. The two stationary gears 3a and 3c are spaced apart and rotatably supported on a base body member 7. The idle gear 3c is rotatably supported on a support member 6. The idle gear 3c can be moved in an engaging direction toward the stationary gears 3a and 3c and can be moved in a disengaging direction away from the stationary gears 3a and 3b. 
The support member 6 is fastened to the base body member 7 with a bolt 8. The bolt 8 is screwed in a threaded hole formed in the base body member 7. The bolt 8 screwed in the threaded hole of the base body member 7 moves in an axial direction 9 to press the support member 6 against the base body member 7. When the bolt 8 is unscrewed, the bolt 8 moves away from the base body member 7 to remove pressure applied to the support member 6. When the support member 6 is fastened to the base body member 7, the support member 6 is restrained from moving in either of the directions 10a and 10b by a frictional resistance. When the support member 6 is unfastened, the support member 6 is allowed to move either of the directions 10a and 10b. 
The operator unfastens the bolt 8, moves the support member 6 in either of the directions 10 to adjust the position of the idle gear 3c relative to the stationary gears 3a and 3b. Then, the operator fastens the bolt 8 to press the support member 6 against the base body member 7. Thus the idle gear 3c is kept in a correct positional relation with the stationary gears 3a and 3b. 
After the bolt 8 has been fastened, a frictional resistance restrains the support member 6 from movement in the directions 10a and 10b. If a force exceeding the frictional resistance acts on the support member 6, the support member 6 is moved in the direction 10a or 10b and, consequently, the idle gear 3c is displaced from a correct position.
Even if the frictional resistance acting on the support member 6 is very large, impact forces often exceed the frictional resistance. For example, an impact force, which is exerted on the stationary gear 3b when the robot arm collides against an obstacle, causes the instantaneous angular displacement of the stationary gear 3b. Consequently, an impact force exceeding the frictional resistance acts instantaneously through the idle gear 3c on the support member 6 to move the support member 6 in the direction 10b. If the idle gear 3c is thus dislocated from the correct position due to the displacement of the support member 6 in the direction 10b, the position of the robot arm cannot be accurately controlled.
Such a problem also arises in machines other than robots having a gear train. The conventional gear train restrains the displacement of the support member by the frictional resistance. Therefore, if a force exceeding the frictional resistance acts on the support member, the center distance between the meshed gears changes and the gears cannot be kept at the desired center distance from each other.